Unwanted hole in metal sheet that use for vehicle body or structure is proved dangerous situation. To prevent disaster this hole needs to be found before its installation, or other wise it will be time and money consuming once its found when already finished installation. Therefore, an inspection using sensor for metal sheet is recommended to prevent this problem. In this paper, we proposed a new sensor using radio wave propagation to detect holes in metal sheet. We propose to use RSSI methods to detect hole based on electromagnetic wave propagation signal strength. Using this method we success to detect 8 mm hole diameter in metal sheet with 1mm thickness. Using this method, we transmit electromagnetic wave energy at about 20 dBm, and we receive with average -27.53 dBm for iron sheet and -23.13 dBm for aluminum sheet.

1. TELKOMNIKA, Vol.17, No.5, October 2019, pp.2481~2485
ISSN: 1693-6930, accredited First Grade by Kemenristekdikti, Decree No: 21/E/KPT/2018
DOI: 10.12928/TELKOMNIKA.v17i5.12684  2481
Received October 27, 2018; Revised March 19, 2019; Accepted April 22, 2019
A low cost electromagnetic sensor for detecting holes
in metallic sheet
Galang P. N. Hakim
1
, Ahmad Firdausi
2
, Mudrik Alaydrus*
3
Department of Electrical Engineering, Universitas Mercu Buana,
Jalan Meruya Selatan, Jakarta, Indonesia
*Corresponding author, e-mail: macross.galang.pes@gmail.com
1
,
ahmad.firdausi@ymail.com
2
, mudrikalaydrus@mercubuana.ac.id
3
Abstract
Unwanted hole in metal sheet that use for vehicle body or structure is proved dangerous situation.
To prevent disaster this hole needs to be found before its installation, or other wise it will be time and
money consuming once its found when already finished installation. Therefore, an inspection using sensor
for metal sheet is recommended to prevent this problem. In this paper, we proposed a new sensor using
radio wave propagation to detect holes in metal sheet. We propose to use RSSI methods to detect hole
based on electromagnetic wave propagation signal strength. Using this method we success to detect
8 mm hole diameter in metal sheet with 1mm thickness. Using this method, we transmit electromagnetic
wave energy at about 20 dBm, and we receive with average -27.53 dBm for iron sheet and -23.13 dBm for
aluminum sheet.
Keywords: electromagnetic wave, holey metallic sheet, sensor, shielding effectiveness
Copyright © 2019 Universitas Ahmad Dahlan. All rights reserved.
1. Introduction
In modern time like this, metal material is one of the most important components that
support human civilization. Almost every device has metal material inside. That metal material
works either as electronic conductor because of their characteristic to deliver electron, or to
become casing because of their strength and durability. Metal sheet as one of many form of
metal material, played a vital role for most today application. Almost every vehicle and
structure [1] has metal material to become support or become their body to cover and thus
provide safety for passenger inside.
The strength and durability in metal sheet also had their flaw. Processing metal raw
material to become metal sheet, sometimes make fatigue for metal sheet, and thus develop
become holes [2]. This hole problem can develop become disaster that threaten life, and proved
fatal in special vehicle that use metal sheet for its body cover (such as submarine and ships).
Such disaster happen in 2008 when Russian spacecraft “Soyuz” experiencing the same
problem [3]. The “Soyuz” has hole on its body with only two millimeters wide, but this hole
causes oxygen to leak into space and thus pressure were dropping inside and threatening the
life of astronouts. Therefore a manual inspection using human eye for metal sheet is a must to
prevent this disaster from happen again [4]. But another problem arises when we are using
human eye to inspect hole in metal sheet. It was unreliable inspection, because human eye
sometimes misses to inspect a very small hole [5].
In this paper, a non-intrusive sensor based on electromagnetic wave is proposed to
solve this problem. Several researches have been conducted for this problem, such as using
RFID [6-8], using resonator and transformer [9-11], using microstrip and waveguide
antenna [12,13]. Other unique methods would be the use of computer vision such as
Otsu-Canny Edge Detection Algorithm [14]. Table 1 shows matrix related research.
Unfortunately, using RFID system, we need additional readers to calculate RFID tag energy
therefore it was costly with range $500 to $3000. Because it was expensive using RFID,
therefore our works was not based on RFID, but rather using an electromagnetic wave
propagation through material and signal measurement [15-17]. An electromagnetic wave is
excited trough metal sheet and if there are hole, it can propagate trough hole. In this work a real
time measurement of received signal strength is carried out, and by comparing with a calibration
value, the shielding effectiveness of the structure is predicted.

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TELKOMNIKA Vol. 17, No. 5, October 2019: 2481-2485
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Table 1. Matrix of Related Research
Publication Application Device
R. Zoughi & S. Kharkovsky [12]
Microwave and millimetre wave sensors for crack
detection
Waveguide Antenna
B. Zhou & J. Zhang [15] Potential Measurement in ECT System Capacitance Electrode
P. Lopato & M. Herbko [16]
Microwave Structural Health Monitoring Sensor for
Deformation Measurement of Bended Steel
Microstrip Antenna
M. Lisowski & T.Uhl [17]
Wireless passive RFID-based sensor for crack
detection
RFID Printed Sheet
This work
A Low Cost Electromagnetic Sensor for Detecting
Holes in Metallic Sheet
ESP8266
2. Measurement Method
The metallic sheet under test is located between the transmitter and receiver, as
illustrated in Figure 1. The transmitter generates the electromagnetic signal with the frequency
2.4 GHz, which is transmitted by the antenna and propagates towards the metallic sheet.
A receiver captures the electromagnetic signals penetrating through the hole. The received
power can reveal the dimension of the holes under certain calibration.
Figure 1. Measurement Method
The designed electromagnetic sensor consists of the transmitter and receiver block.
The transmitter generates an electromagnetic wave which we tag with “SSID Metal Check”.
The receiver catches the electromagnetic wave signal, and using Arduino IDE Serial Monitor
calculates the received signal strength. To keep the interference to the minimal level, we placed
the transmitter block inside a 0.5 mm thick metallic case and the receiver block on a plastic
case. The microcontroller Wemos (esp8266 based) is used as an electromagnetic wave signal
transmitter and also as receiver. The microcontroller Wemos is a popular IoT node, which is low
cost and has compact size. It is a programmable microcontroller with L106 32-bit RISC
microprocessor core based on the Tensilica Xtensa Diamond Standard 106 Micro running at
80 MHz. This microprocessor has also a WiFi capability with an integrated TR switch, balun,
LNA, power amplifier and matching network.
The microcontroller wemos is powered by Li-Po battery; the battery charger and boost
converter for recharging the battery and powering the entire system; see Figure 2.
The transmitter circuit consists of four blockes, microcontroller wemos (A), the battery (B),
battery charger and boost converter (C) and the on/off switch (D), as given in the left side of
Figure 2. For the transmitter it is essential to set the equivalent isotrop radiated power (EIRP) in
order to be able to provide detectable received level behind the hole. The EIRP of
the transmitter consists of the transmit power (PTX) and the gain of the transmitting
antenna (GTX):
𝐸𝐼𝑅𝑃 = 𝑃𝑇𝑋 + 𝐺 𝑇𝑋 (1)
PTX = transmit power (17dBm)
GTX = gain of the antenna (3 dBi)

3. TELKOMNIKA ISSN: 1693-6930 
A low cost electromagnetic sensor for detecting holes in metallic sheet (Galang P. N. Hakim)
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The EIRP becomes 17 dBm + 3 dBi = 20 dBm or 100 mW.
Whereas as illustrated in the right side of Figure 2, the receiver is also composed of
microcontroller wemos (A), the battery (B), battery charger and boost converter (C) and the on/off
switch (D). The received power can be calculated by (2):
𝑃𝑅𝑋 = 𝐸𝐼𝑅𝑃 − 𝑃𝐿 ≥ 𝑃𝑅𝑋,𝑚𝑖𝑛 (2)
PRX = Received Power (dBm)
PL = Path loss (dB)
PRX,min = minimal received power/sensitivity of the receiver (-98 dBm)
(a) (b)
Figure 2. (a) Transmitter and (b) receiver circuitry
In path loss we include the geometrical power decrease, all possible disturbances along
the propagation path and the gain of the receiving antenna. By this occasion, the effect of
the hole in metallic sheets can be described in the penetration of the fields through the hole.
The additional loss due to the hole is calculated in a simple way by comparing the received
power in the presence of the hole to the case without the hole. As long as, the value of
the received power is higher than the minimal detectable received power, we can measure
meaningful received power.
Moreover, there is a shielding characteristics due to the presence of the holey
sheet [18], which can be quantified by its shielding effectiveness. For a circular hole in a
metallic sheet, a simple equation is available as given in [19]:
𝑆𝐸 = 20 log (
𝜆 𝑜
2𝑑
) (3)
𝜆 𝑜 is the wavelength in the free space and d is the diameter of the hole.
In this work, we can calculate the shielding effectiveness simply by taking the difference
between the received power without and with the holey sheets:
𝑆𝐸 = 𝑃𝑅𝑋,𝑊𝑖𝑡ℎ𝑜𝑢𝑡 − 𝑃𝑅𝑋,𝑤𝑖𝑡ℎ (4)
3. Metallic Sheets under Test
In this work, we use two different metal specimens; the aluminum sheet and iron sheet.
Aluminum is a strong and light weight metal that can be found in everyday life such as food
wrapping, furniture, panel [20] etc. More importantly, aluminum is used in most aircraft [21], use
for shielding against lightning strike [22], also because its metal feature provides durability
against weather (corrosion resistant) [23] and has good thermal conductivity [24]. Meanwhile,
the iron can easily be found in a heavy duty application such as bridges [25], roads, and others.
In this experiment, we use iron and aluminum metal sheets with 1 mm thickness and
the dimension 10 cm x 15 cm. Each aluminum and iron metal sheet we built consist of a hole
with diameters of 2 mm, 4 mm, 6 mm and 8 mm, as given in Figure 3.

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4. Measurement
A real time measurement is carried out to validate whether this sensor can detect a hole
in the metal sheet. In order to do that, at first we are going to measure the received power
without metal sheet. This measurement becomes a basis for calibration purposes for other
measurements. Any variation of the received power means disturbances in the propagation
path, which in this work is a metallic sheet with a hole with different diameter. Table 2 shows
the measured received signal strength for free space condition (without metal) and for the case
metallic sheets with different hole diameters. Based on the measured received power in
Table 2, the shielding effectiveness of holey iron and aluminum sheet is calculated with (4), and
for comparison purposes we use also the analytic equation in (3). The results are given
in Figure 4.
Figure 4 reveals that iron shows bigger shielding effectiveness than aluminum sheet
has. As the hole diameter increases, the value of shielding effectiveness decreases, as also
given by the simple analytical expression. We see the order of the measured values is the same
like given in the analytical approach.
Figure 3. Aluminum & iron metal sheet
specimens, from left to right (hole with a diameter
of 2 mm, 4 mm, 6 mm and 8 mm)
Figure 4. Shielding effectiveness of holey
iron and aluminum sheets
Table 2. RSSI Measurement obtained for several metallic sheets (values in dBm)
Spesimens
Without
Metal
Metal with hole, diameter
2 mm 4 mm 6 mm 8 mm
Iron -4.6 -33.1 -32.0 -34.1 -27.5
Aluminum -4.6 -27.3 -23.6 -26.1 -23.1
5. Conclusion
In this research we proposed a low cost electromagnetic wave based sensor for hole
detection in metal sheet. This sensor only cost about $37.8 dollar more cheaper compare with
RFID system based sensor. Using RSSI method to simplify the real time measurement, we
success to detect 8 mm hole in diameter in 1 mm thickness metal sheet. The transmitted signal
strength was 20 dBm and the received signal strength at average -27.53 dBm for iron sheet
and -23.13 dBm for aluminum sheet. We were also able to predict the shielding effectiveness of
the holes with different diameters and compared the result with those found by
analytical expression.
Acknowledgements
The authors would like to thank Kemenristek Dikti for the financial support of this
research for the period 2019.

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